Shortly after World War II the first aerosol dispensers were introduced. Propane was the propellant and it delivered insecticide. The use of propane required a container with a burst pressure near 300 pounds per square inch (psi) since the vapor pressure of propane at room temperature is about 190 psi, a relatively high pressure, and in many ways unsafe, but tolerable in its time.
About 1955 the market for this aerosol method of dispensing began to increase rapidly with the introduction of chlorinated fluorocarbons (CFCs) which were regarded as an ideal propellant. CFCs are non-flammable, non-toxic, relatively inexpensive, capable of a wide range of propellant formulations to give any desired vapor pressure, and soluble in a wide variety of product formulations.
Aerosol spray containers soon became widely used to deliver a broad range of materials of various compositions and properties to the point of use. Products which employ this system are numerous and include such products as hair sprays, colognes, tanning aids, medicinals, waxes and polishes, foods, adhesives and paints.
In the 1970s it became believed that a build-up of CFCs in the atmosphere was damaging the ozone layer which surrounds the earth. This envelope absorbs a great deal of the sun's radiation in the ultra-violet region. Ultra-violet radiation can cause human skin cancer, eye diseases and other ailments, and can also lead to increased damage to crops and fish.
CFCs are produced from non-aerosol applications as well; however, the environmental impact of CFCs in these applications is uneven since they are subjected to varying levels of containment and control. Aerosols, however, are the only application where the CFC is deliberately expelled to the atmosphere with no attempt at scavenging or recycling. This, together with the availability of alternative dispensing methods, made this application a prime target for legislative control, once the severity of the environmental impact became clear.
In 1979, the U.S. government passed laws providing for the elimination of CFCs in aerosol dispensers. Recently, other countries have moved to curtail, and eventually prohibit, their production.
In addition to their possible impact on the ozone layer, it is now believed that CFCs are also significant contributors to the greenhouse effect (a long-term warming trend in the earth's atmosphere). The search for alternatives to CFCs has been difficult.
To be useful, a propellant should have a vapor pressure higher than 10-20 psi at room temperature so that it will be effective over the full range of product viscosities; from perfumes at one end of the range to shaving gels at the other. At the same time, because the can will sometimes become overheated thus causing pressure increase, the propellant must be contained at temperatures up to 120.degree. F. without making the container so heavy as to be unwieldy. And of course, costs must be as low as possible.
Systems were developed to generate gas by chemical reaction on an as-needed basis. One such system consists of a can within which is a flexible membrane holding the product to be dispensed. In the bottom of the can is a liquid which is one component of the binary mixture needed to generate gas. The membrane is fastened to the wall of the can with an adhesive which contains the other component needed for the gas-generating reaction. As the product is dispensed, the membrane collapses upward, exposing the adhesive to the liquid in the bottom of the can and generating more gas. Once dispensing stops the pressure builds up, compressing the unused product until no more fresh adhesive is exposed and the gas-generating reaction is thereby stopped. This system has never received wide-spread use, chiefly because of practical problems. When the can is laid on its side, the membrane can sag a bit, exposing more adhesive and generating more gas. Eventually, the reaction makes its way up the side of the can until either the can's burst pressure is exceeded, or product is trapped below a zone of high pressure in the top of the can and cannot be dispensed. A number of patents have issued in the area, despite the fact that such systems, in general, have not been commercially practical. See e.g., U.S. Pats. 4,896,794; 4,857,029; 4,696,145; 4,679,706; 4,641,485; 4,621,483; 4,611,457; 4,594,834; 4,553,685; 4,531,341; 4,513,884; 4,510,734; 4,478,044; 4,376,500; and 4,360,131.
A more recent invention relies on the elasticity of rubber to provide the propelling force. A vertically pleated flexible bottle is fitted with a valve, and then inserted inside a rubber sleeve. This assembly is placed inside a retaining display container and sealed. Product is forced past the valve and into the flexible bottle, expanding it and the surrounding rubber sleeve. In this way, energy is stored in the rubber sleeve which expels the product when the valve atop the flexible bottle is opened. This system is limited in its capabilities by the strength and resilience of the rubber sleeve. There is also concern that the elastomer used in the flexible bottle and the rubber sleeve may deteriorate in contact with some products and thus exhibit limited shelf life. Such a system is described in U.S. Pat. No. 4,964,540.
Organic gases were considered and eventually yielded useful propellants but, as a class, these materials suffer from some important disadvantages. Those organic gases that have vapor pressures suitable for propellant use at room temperature have rather steep pressure gradients, so that the internal pressure at 120.degree. F. is relatively high, requiring a stronger and more expensive container than would otherwise be the case. In addition, most organic gases are flammable and form explosive mixtures with air. Another disadvantage is that many organic propellants, such as isobutane, readily diffuse through polymers, especially those of a thermo-formed nature, with the result that the use of plastic as a container material is largely precluded, strength and safety issues aside. Finally, many organic propellants have a latent heat of vaporization and other thermodynamic characteristics such that the temperature of the aerosolized product stream is lowered significantly below ambient, thus reducing the efficiency of aerosolization and requiring a high ratio of propellant to product.
Among the organic gases, isobutane and propane were the most suitable. Mixtures of these gases permitted a range of vapor pressure to meet the needs imposed by widely varying product viscosity. Even so, they presented some serious problems. At room temperature isobutane has a vapor pressure of about 32 psi. This is quite high and led to problems in closing the container. For a time, the problem was addressed by refrigerating the filling environment. This lowered the vapor pressure of the propellant and facilitated can closing, but was expensive. Currently, filling is accomplished principally by forcing the propellant mixture in through the valve. Effective but also expensive, in that it requires special filling equipment. In addition, at 120.degree. F., the vapor pressure of isobutane is 92 psi. This is a fairly high pressure for a hand-held consumer product and the container must have adequate strength. Designed burst pressure is 130 psi. These factors made it very difficult to design a plastic container that would be safe and cost effective. Steel has an adequate strength-to-weight ratio and is relatively inexpensive but cannot be adequately deep drawn, with the result that the top, bottom and side seams must all be welded, adding to cost. Aluminum can be deep drawn so only the top must be welded, but the material itself is relatively expensive. With either material, the cost of adequate containment is substantially higher than with CFCs.
Finally, isobutane and propane are both flammable and form explosive mixtures with air. This led to two expensive consequences. First, the filling equipment and the filling room had to be made explosion-proof, at considerable expense. For some producers, the cost was prohibitive and the introduction of these propellants led to a substantial growth in those companies which do contract filling; a very unsatisfactory condition for affected producers who were now required, for economic reasons, to lose control of a part of the manufacturing process, while their name remained on the package. Second, product liability risks were greatly increased at every step of manufacturing, distribution and use.
Clearly, the successors to CFCs have required some very substantial trade-offs in terms of cost, safety and effectiveness.
There are many non-aerosol applications where product dispensing utilizing gas pressure would be desirable. For example, in traditional deodorant systems, a wick is immersed in a deodorant solution. A small battery-powered fan blowing air over the wick may be used to help evaporate the solution. Energy consumption for the fan motor is high so frequent battery replacement is needed. Also, operation is continuous so deodorant is dispensed whether the facilities are in use or not. A spring-wound device which periodically depresses the actuator of an aerosol dispenser, releasing deodorant into the room may be utilized in place of the wick. However, the need to rewind the timing device makes this product somewhat inconvenient to use.
Therefore, there still exists a need for gas generation cell which is a safe, practical and environmentally acceptable means for product dispensing using gas, whether in the form of an aerosol or otherwise.